mcast.go

// Add multicast support to UDPConn
package net

import (
	"os"
	"syscall"
	"fmt"
)

// Join a multicast group. Only IPv4 addresses are supported
func (c *UDPConn) JoinGroup(addr IP) os.Error {
	ip := addr.To4()
	if ip == nil {
		return fmt.Errorf("%s is not an IPv4 multicast address", addr)
	}
	return os.NewSyscallError("setsockopt", syscall.SetsockoptString(c.fd.sysfd, syscall.IPPROTO_IP, syscall.IP_ADD_MEMBERSHIP, makeMReqString(ip)))
}

// Leave a multicast group. Only IPv4 addresses are supported
func (c *UDPConn) LeaveGroup(addr IP) os.Error {
	ip := addr.To4()
	if ip == nil {
		return fmt.Errorf("%s is not an IPv4 multicast address", addr)
	}
	return os.NewSyscallError("setsockopt", syscall.SetsockoptString(c.fd.sysfd, syscall.IPPROTO_IP, syscall.IP_DROP_MEMBERSHIP, makeMReqString(ip)))
}

// construct a string to pass to syscall.SetsocketoptString
// that conforms to the ip_mreq struct. addr must be an IPv4 
// style address
func makeMReqString(addr IP) string {
	return string(append([]byte(addr), 0, 0, 0, 0))
}

mulitcast_test.go

package main

import (
	"net"
	"log"
	godns "github.com/miekg/godns"
)

func main() {
	addr, err := net.ResolveUDPAddr("0.0.0.0:5353")
	if err != nil {
		log.Exit(err)
	}
	socket, err := net.ListenUDP("udp", addr)
	if err != nil {
		log.Exitf("listen %s", err)
	}
	defer socket.Close()

	err = socket.JoinGroup(net.IPv4(224, 0, 0, 251))
	if err != nil {
		log.Exitf("join group %s", err)
	}
	for {
		buff := make([]byte, 4096)
		read, err := socket.Read(buff)
		if err != nil {
			log.Exitf("read %s", err)
		}
		msg := &godns.Msg{}
		msg.Unpack(buff[:read])
		log.Println(msg.String())
	}
}